Similar to 495af07 ("bpf: Fix csum logic in snat_v4_rewrite_headers"),
we mustn't mix the diff from the L4 port-rewrite into the diff used for the
pseudo-header update.

Signed-off-by: Julian Wiedmann <jwi@isovalent.com>

When a local endpoint sends an ICMP_FRAG_NEEDED message, it contains some
inner packet that was originally addressed to the endpoint. The endpoint's
address/port in this inner packet potentially needs to be SNATed again, to
return it to its pre-revSNAT state.

snat_v4_icmp_rewrite_egress_embedded() handles the .daddr / .dport rewrite
of such an inner packet, based on an SNAT entry. Replace it with the
common snat_v4_rewrite_headers() helper.

Signed-off-by: Julian Wiedmann <jwi@isovalent.com>

Use the common snat_v6_rewrite_headers() helper from the IPv6 RevSNAT path.
This is essentially a 1-to-1 replacement, except that we need to ensure
that the L4 port rewrite is skipped for ICMPV6_PKT_TOOBIG packets.

Signed-off-by: Julian Wiedmann <jwi@isovalent.com>

When loading the ICMP identifier field, store it in-place instead of going
through a temporary variable.

Also as the `tuple` is zero-initialized, we don't need to do this again for
the port field(s).

Signed-off-by: Julian Wiedmann <jwi@isovalent.com>